This invention relates generally to the welding of thin metal parts and particularly to the spot welding of the metal parts of an electron gun assembly for color picture tubes.
In the spot welding of thin metal parts, a welding electrode is brought into contact with one side of the parts to be welded and a ground electrode is brought into contact with the opposite side of the parts. A voltage is placed on the welding electrode and discharged from the welding electrode, through the parts and to the ground electrode. The current passing through the metal melts a small area of the metal where the welding electrode contacts the part. When the molten metal cools, the parts are blended, or welded together. Typically, the voltage needed on the welding electrode is provided by charging a capacitor which stores the voltage until the discharge through the parts to be welded is effected. Typically, the capacitor is charged using a fixed voltage source and a timing means. Accordingly, the capacitor is charged for a fixed period of time without regard to the actual voltage to which it is charged. Variations in contact resistance or other parameters in the charging system frequently cause the voltage to which the capacitor is charged to vary because the parameter variations result in changes in the r-c time constant of the charging network. The charging voltage variations result in inconsistency in the welding of the thin metal parts because the welding current is directly dependent upon the charge on the storage device. There, therefore, is a need for a system for repeatedly and accurately charging the storage device to a particular voltage to improve the consistency in the welding of thin metal parts.
The instant invention fulfills this need by the provision of a system and method for accurately controlling the electrode voltage of a welding device independently of time and of resistance variations of the welding or charging systems.